\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide}
\usepackage{amsmath}
\usepackage{wasysym}
\usepackage{circuitikz}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes}
\ctikzset{bipoles/length=1cm}
\begin{document}
\small
\begin{circuitikz}[>=latex, scale=1,european]
  % \useasboundingbox(0.9,0)rectangle(5.1,5);
  \draw (0,0) to [R=$R_1$] (2,0) to [R=$R_2$] (4,0) to [R=$R_3$] (6,0);
  \draw [->](-.5,0)--(.25,0)node[above]{$I$} ;  
  \draw [->](2,0)--(2.25,0)node[above]{$I$};     
  \draw [->](4,0)--(4.25,0)node[above]{$I$};
  \draw [->](5.75,0)--(6.5,0)node[above]{$I$};
  \draw[<->] (.25,-.75)--node[fill=white]{$U_1$} (2.25,-.75);
  \draw[<->] (2.25,-.75)--node[fill=white]{$U_2$}(4.25,-.75);
  \draw[<->] (4.25,-.75)--node[fill=white]{$U_3$}(6.5,-.75);
  \foreach \x in {.25,  6.5}
  {
      \draw (\x,-.2)--(\x, -1.8);
  }
  \draw[<->] (.25,-1.5)--node[fill=white]{$U$}(6.5,-1.5);
  \foreach \x in { 2.25, 4.25}
  {
      \draw (\x,-.2)--(\x, -1.2);
  }
\end{circuitikz}
\end{document}